Dense sintered articles comprising polycrystalline silicon nitride which have been produced with the concomitant use of sintering additives based on oxides are known. These articles have high strength at room temperature, high resistance to fracture, low thermal expansion and very good resistance to wear. These sintered articles are manufactured from silicon nitride powders, with sintering additives, according to known sintering processes with and without the use of pressure. As sintering additives, those based on oxides such as magnesium oxide, yttrium oxide and other oxides of rare earths are especially useful. They are used individually or as mixtures or in the form of pre-sintered mixed oxides of the spinel type. Depending on the type and amount of sintering additives and the particular sintering process used, dense sintered articles can be obtained. These articles generally have good or very good bending strengths at room temperature (Cf. U.S. Pat. No. 3,950,464, U.S. Pat. No. 4,004,937 and U.S. Pat. No. 4,180,410, which corresponds to DE-C No. 28 20 694).
It is also known, that sintered articles based on silicon nitride manufactured using these known oxides or mixed oxides as sintering additives generally have poor resistance to high temperature. Their bending strength at about 1300.degree. C. is less than 50% of the value measured at room temperature. This decrease in bending strength has been attributed to the presence of non-crystalline, glass-like phases, at the grain boundaries.
Since poor physical properties at high temperatures severely restrict the use of the sintered articles as components which are subject to stress at high temperatures, numerous proposals have been made to prevent the formation of these undesirable intergranular glass-like phases to or subsequently convert these glass-like phases into crystalline phases.
For example, it is known that the formation of crystalline phases comprising Si.sub.3 N.sub.4.nY.sub.2 O.sub.3, in which n represents 1, 2 and/or 3, can be obtained by sintering pre-compressed articles comprising silicon nitride and yttrium oxide powder in the presence of aluminum nitride, with and/or without the use of pressure, at temperatures of from 1650.degree. to 1800.degree. C. The purity of the silicon nitride starting powder used was not critical in this instance, that is, small amounts of impurities such as iron, aluminum or calcium, can be tolerated. The amounts of yttrium oxide added should, however, preferably be less than 5 percent by weight (cf. U.S. Pat. No. 4,046,580, which corresponds substantially to DE-B No. 25 28 869).
Another method was disclosed in U.S. Pat. No. 4,102,698, in which silicon nitride, yttrium oxide and silicon dioxide powder mixtures of a specific composition were sintered with the use of pressure. The amounts of the materials used corresponded to a specific area in the ternary system Si.sub.3 N.sub.4 -Y.sub.2 O.sub.3 -SiO.sub.2, to exclude the possibility of the formation of unstable compounds. The data shows a considerable decrease in the bending strength between room temperature and 1400.degree. C.
A further improvement in high temperature properties was disclosed as being obtained by using very pure powder mixtures of specific composition in amounts which corresponded to a specific area in the ternary system Si.sub.3 N.sub.4 -SiO.sub.2 -M.sub.2 O.sub.3, wherein M comprises at least two different components each of which have an ionic radius of less than 0.89 A. The sintered articles are prepared by hot pressing or isostatic hot pressing mixtures of the individual oxides, together with Si.sub.3 N.sub.4 powder. The oxides can, also be melted to form a pyrosilicate which is then mixed with the Si.sub.3 N.sub.4 powder before the hot-pressing operation (cf. U.S. Pat. No. 4,234,343).
In order to facilitate the formation of a crystalline phase instead of the undesired glass-like intergranular phases, it is known to concomitantly use small amounts of nucleus formers such as iron and titanium, in elementary form or in the form of oxides and nitrides (cf. U.S. Pat. No. 4,179,301), or tungsten carbide, titanium carbide and aluminum oxide, which can be introduced in the form of abraded material from grinding during the manufacture of the homogeneous powder mixture (cf. U.S. Pat. No. 4,401,617 and EP-A No. 9859). The introduction of the sintering additives in the form of abraded material from grinding using grinding bodies comprising an oxide or a silicate is also known (cf. DE-A No. 28 00 174).
In addition, attempts have been made to use instead of yttrium oxide, which is generally regarded as one of the most effective sintering additives, a high-melting zirconium compound, zirconium silicate (cf. U.S. Pat. No. 4,069,059). In addition, it is said that the physical properties of sintered articles comprising silicon nitride can be improved by the concomitant use of lithium oxide or aluminium oxide and iron oxide as sintering additives in addition to yttrium oxide. Attention is drawn to the fact that by the specific addition of these constituents, drastic deterioration of long-term and short-term properties, due to oxidation, can be avoided, both at room temperature and at high temperatures; (cf. EP-A No. 45518).
It is clear from the extensive prior art that the formation of secondary crystalline intergranular phases instead of glass-like phases, is necessary to obtain good high-temperature properties during the sintering of silicon nitride. The prior art teaches that specific measures such as heat treatment in the presence of aluminium nitride, a limited choice in regard to the type and amount of sintering additives used, the use of very pure silicon nitride starting powders or the concomitant use of nucleus formers or other substances is necessary to obtain good high temperature properties. Apart from the fact that some of the individual measures are expensive, they do not guarantee that the finished sintered articles based on silicon nitride will have the following combination of properties:
(1) high density, PA1 (2) high mechanical strength at room temperature and a small decrease in strength with temperature and PA1 (3) a long life that is, a slow decrease in strength with time. PA1 CaO.ltoreq.0.4% by weight PA1 MgO.ltoreq.0.2 by weight PA1 Na.sub.2 O.ltoreq.0.1 by weight PA1 K.sub.2 O.ltoreq.0.1 by weight PA1 Li.sub.2 O.ltoreq.0.1 by weight
The problem is to provide polycrystalline sintered articles based on silicon nitride and sintering additives which have this combination of properties.